Portable electrotherapy devices typically utilize a relatively small battery to power internal circuitry which, in turn, provides an output in the form of an electrical signal. The electrical signal output of such devices has been shown to have therapeutic benefit to a patient for a wide variety of medical conditions. These devices have been described as non-invasive nerve stimulation devices, electro-acupuncture devices, and in acupuncture terminology stimulate an acupuncture point.
Portable electrotherapy devices are most conveniently powered by coin cell or button cell batteries such as those used for hearing aids, calculators and other small consumer electronic devices. Through pulse generating circuits, these devices may deliver thousands of pulses per hour from the battery. For example, an electrical pulse repetition rate of approximately 70 pulses per second and a pulse width of 80 microseconds have been found to provide effective relief of nausea. Hundreds of thousands to millions of pulses may be required to treat long lasting nausea, as may be required for post operative nausea, chemo-therapy nausea and other long lasting nausea conditions.
These electrotherapy devices provide a variety of pulse amplitude variations and combinations, or bursts of pulses at specific intervals. For example, the electrical pulse pattern used in our ReliefBand® electrotherapy product comprises about 350 microsecond pulse width at about 31 pulses per second at power levels of about 10–35 milliamps peak pulse height. A wide range of pulse patterns may be used in noninvasive nerve stimulation devices.
Other electrotherapy devices are designed for Functional Electrical Stimulation (FES), which exercises muscles near the point of stimulation. Transcutaneous Electrical Nerve Stimulation (TENS) inhibits sensory nerve communications in the area of stimulation to block pain. All these battery powered electrotherapy devices are characterized by a stimulation output, typically in the form of a voltage or current pulse delivered at a particular pulse shape and waveform. The pulse amplitude, pulse width, and pulse frequency are selected so as to be suitable for treating particular symptoms or conditions such as pain, addiction, nerve disorder, muscle disorder, organ malfunction, etc. Patients using these devices receive direct benefit through the improvement of their quality of life.
The energy needed to deliver the stimulation output is delivered from a battery supply, which may consist of one or more battery cells at a particular nominal voltage and particular battery chemistry. Our ReliefBand® electrotherapy device uses coin cell batteries of standard size, which are readily available. The stimulation output peak pulse amplitude is commonly in a range of 1 to 100 milliamps, which is delivered for a particular time depending on the pulse waveform.
Many types of batteries suitable for use in battery powered electrotherapy devices are optimized to deliver electrical current at lower loads than the required stimulation output. For example, a typical coin cell battery may be rated to provide 0.1 to 0.3 milliamps of current for 100 minutes if the battery is drawn down at an average electrical current draw of 0.1 to 0.3 milliamps.
As a result of the discrepancy between the optimal current draw on the battery and the current draw required for therapeutic pulses, the battery is not used optimally and battery performance and battery life are degraded. Because of battery chemistry, the overall amount of power that can be drawn from a battery is smaller for large current drains than for small current drains. A battery may be able to provide 0.02 milliamps for 100 minutes, but may only provide current of 0.1 milliamps for 10 minutes (instead of 20 minutes), so that half the battery power is lost if the current is drawn off rapidly. Moreover, the problem becomes even greater as the current draw is increased. Thus, drawing current at the rate of 1 milliamp will not provide the expected 1 minute of current (at an expected half power loss), but will provide far less, perhaps only a small fraction of a minute of current at 1 milliamp. For example, if a battery is discharged for a 10 millisecond pulse of 1 milliamp every second, the average current draw is 0.01 milliamps, but the battery will be depleted according to the instantaneous current of 1 milliamp, not the average current of 0.01 milliamps. Rather than obtaining 100 minutes of operation, the battery will provide far less current and power. If, however, the battery is discharged at 0.02 milliamps for a 0.5 second pulse every second, the average current draw still is 0.01 milliamps, but the battery will last according to the instantaneous current draw of 0.02 milliamps. The battery will provide 100 minutes of current when drawn down in this manner.
Battery powered electrotherapy devices usually require a higher voltage therapeutic output pulses than can be provided by conveniently available batteries. Accordingly, electrotherapy devices typically use a transformer to step up the pulses from the battery output to the higher voltage output required for therapeutic devices. The high voltage output is required to allow the pulsed electrical current to be delivered to a particular electrical load, for example, living tissue. The electrical impedance of human skin can be modeled as a 500 ohm resistance. Accordingly, if the device is to deliver 30 milliamps into the skin, then it needs to provide a 15 volt output across the skin.
In a conventional electrotherapy device, a transformer is typically connected to the battery, either directly or through a switching mechanism, and the voltage output from the transformer is proportional to the battery voltage. A problem occurs when the battery voltage begins to lower as the battery becomes depleted. Because the high voltage output is proportional to the battery voltage, the output voltage capability lowers and eventually the electrical current output also lowers for a given electrical load. When the current output lowers, the device's therapeutic effectiveness is lessened.
This problem is a serious problem for patients who use electrotherapy devices for chronic conditions. The patient may experience a lower quality of life, and possibly a degradation in health, as the device's therapeutic effectiveness diminishes over time. The device may provide a low battery indicator, but effectiveness may still be diminished. The device may also just shut off if the battery becomes too depleted, at which point the individual is left without the therapeutic benefit of the device with no adequate warning to allow for a replacement device or battery supply to be obtained. Moreover, current electrotherapy devices do not manage battery consumption so as to obtain the maximum available amount of power from the battery. This leads to more frequent battery replacements than would be required if the battery power could be managed more effectively.
Various circuits have been proposed for use in monitoring charge remaining on a battery, or to generate a pulse from a battery for use in an electrotherapy device. A number of devices have used methods for measuring remaining battery capacity directly for implanted devices e.g., Renirie et al., U.S. Pat. No. 5,369,364, Schmidt, U.S. Pat. No. 5,369,364, Arai, U.S. Pat. No. 5,744,393, Thompson, U.S. Pat. No. 5,391,193. These methods may include switching to an alternative power source e.g., Fischell, U.S. Pat. No. 4,096,866, or disabling the therapeutic output on a low battery condition, e.g., Putzke, U.S. Pat. No. 4,024,875, but do not address the regulation of the stimulation output as the battery is depleted.
Privas, U.S. Pat. No. 5,218,960, describes a low battery voltage detector for stopping stimulation pulse generation when the battery is too low, but that method requires a priori knowledge of the low battery cutoff voltage so that the circuit component values can be set accordingly. Privas does not address the problem of the therapeutic output voltage lowering as the battery voltage lowers to the cutoff value, thereby decreasing the therapeutic value of the output. Also, Privas does not provide the individual with adequate warning of the pending low voltage condition and cessation of therapeutic output, rather, the output is stopped and the low battery signal is given at the same time.
Dufresne et al., U.S. Pat. No. 4,926,864, describes a circuit for generating a high voltage and monitoring that high voltage through circuit feedback to maintain the high voltage within a specified range as the battery supply is depleted. The Dufresne et al. method suffers in that the charging pulse width in the high voltage generator must be lengthened as the battery supply is depleted. This causes an increase in power consumption that Dufresne et al. address by limiting the charging pulse width to a maximum value. As a consequence, the Dufresne et al. method cannot dynamically adjust the monitored high voltage generator to take advantage of the full range of battery supply capacity. Further, Dufresne et al. makes no provision for adequately warning the patient of the remaining battery life when their control circuit switches to a lengthened pulse width.
Owens, U.S. Pat. No. 5,065,083, describes a system for monitoring the battery voltage and decreasing the output power to allow the system to operate at lower battery voltage as battery power decreases during normal use. The Owens method suffers in that output power must be decreased, rather than maintained at a constant, therapeutic level. Although Owens provides for a low battery indicator, the only indication given is that the output has been decreased. It does not provide for any indication of remaining battery life.